use std::io::Read;

fn main() {
    let mut buf = String::new();
    std::io::stdin().read_to_string(&mut buf).unwrap();
    let mut iter = buf.split_whitespace();

    let n: usize = iter.next().unwrap().parse().unwrap();

    let mut a: Vec<i32> = Vec::with_capacity(n);
    for _ in 0..n {
        let x: i32 = iter.next().unwrap().parse().unwrap();
        a.push(x - 1);
    }

    let mut b: Vec<i32> = vec![-1; n];

    for (i, x) in a.iter().enumerate() {
        let j: usize = *x as usize;
        if b[j] >= 0 && b[j] != (i - 1) as i32 {
            if b[j] == 0 && i == n - 1 {
                println!("1");
            }
            else {
                println!("-1");
            }
            return;
        }
        b[j] = i as i32;
    }
    println!("0");
}